If you're having trouble with the app crashing or not downloading a book, the best thing to do is to delete the app and download it again from the App Store. Sorry about this!
If a specific book you have downloaded to the app is not opening, then you can try removing the book from your device by long pressing on the book cover and deleting it. You should then be able to download it again by tapping on it. Hopefully one of those fixes your issue.
Please note that not all Leanpub books can be viewed in our app. Only books generated using the Leanpub writing and publishing workflow can be viewed in the app. If an author has made a book using their own tools and then uploaded it into our bookstore, then it cannot be read in our app.